Nets protect against malaria, scientists say (but you already knew that)

Global health number crunchers, led by Seattle’s Institute for Health Metrics and Evaluation, have determined after an exhaustive survey of medical and demographic records in 22 sub-Saharan African countries that treated bed nets do protect against malaria.

Duh, you say.

You might well think it’s a no-brainer to ask if handing out hundreds of millions of insecticide-treated bed nets in sub-Saharan Africa helps to reduce malaria.

But it’s not.

To begin with, there are lots of things that can — and do — reduce the incidence of malaria deaths and illnesses. There is the practice of indoor household spraying of insecticide, which has been increased along with the massive campaign to distribute insecticide-treated nets (ITNs).

There also seems to be a routine ebb-and-flow of malaria severity in the tropics. The disease, like many things in nature, tends to alternate between severe and mild cycles. It’s not clear why.

And there was this confusing report recently, about the unexplained decline in mosquito populations in parts of Africa. This happened even in places where nobody was spraying or using bed nets.

There’s changes in rainfall patterns and land use that affect mosquito breeding. There are changes in access to malaria drugs (which has also been increased in recent years). And there’s a chronic problem of misdiagnosis of malaria in poor communities lacking labs.

So, no, it’s not as easy as you think it is to simply attribute a decline in malaria cases with any particular intervention — to simply say that more bed nets means less malaria.

That is basically what Stephen Lim and his colleagues say in their report — Net Effects: A Multicountry Analysis of Observational Data Examining Associations between Insecticide-Treated Mosquito Nets and Health Outcomes — published today in PLoSMedicine.

But Lim and his team had to do a lot of analysis and data refinement to feel comfortable saying this, because of all the other possible explanations and confounding influences on malaria.

“We take these other possible factors into account,” Lim said. Rather than look at population-level data, he said, they drilled down to review what happened in individual households while also reviewing medical records testing for the presence of the malaria parasite (parasitemia).

What this and other methods allowed the researchers to do, Lim said, was reduce the number of other possible explanations for the observed decline in malaria cases and deaths among children under age 5 years.

Overall, Lim and his colleagues found that ITN use reduced child mortality by nearly 25 percent.

Malaria kills mostly young children, millions of them every year, and so changes in child mortality are the best measure of effectiveness. Over the past decade, international funding for malaria control (mostly in Africa) has been increased dramatically to reduce these child deaths. Most of that spending went to distributing hundreds of millions of ITNs.

“But most of the evidence for the effectiveness of ITNs came from relatively small studies,” Lim said. This, he said, was the most comprehensive study of bed net impact to date.

Reports like the mysteriously disappearing mosquitoes or a recent study that warned bed nets might actually be making mosquitoes resistant will still raise questions. There are also questions of sustainability, if donors or governments will support continued distribution as nets wear out.

But for Lim and his colleagues, the evidence clearly supports continuing to scale-up the distribution of ITNs as an effective means to reduce malaria deaths and illness.

“Full steam ahead,” he said.

Note: The study done by Lim and his colleagues was funded by the Bill & Melinda Gates Foundation. The IMHE reports notes that the foundation, which supports the anti-malaria strategy of wide bed net distribution, had no influence over the study design, analysis or publication.

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Tom Paulson

Tom Paulson is founder and lead journalist at Humanosphere. Prior to operating this online news site, he reported on science,  medicine, health policy, aid and development for the Seattle Post-Intelligencer. Contact him at tom[at]humanosphere.org, follow him on Twitter @tompaulson and/or send a comment below.